The present study examines the effect of the serotonin transporter (5-HTT) genotype (LL, Ls, and ss) on rhesus monkey mothers’ treatment of their three-month-old infants—an age when infants are highly dependent on their mothers for security and protection. We hypothesized mothers with the s-allele would exhibit a mothering style characterized by neglectfulness, insensitivity, and aggression. To test this hypothesis, four, 300-second behavioral observations of mothers’ maternal behaviors were recorded for N=69 rhesus macaque mothers (n=29 homozygous for the L-allele, n=11 homozygous for the s-allele, and n=29 Ls heterozygotes). Maternal behaviors recorded included maternal frequency of infant restraints and infant rejections. Results showed that mothers with the s-allele exhibited higher rates of infant rejections (p=0.032) and lower rates of restraint (p=0.012) when compared to homozygous LL mothers, potentially neglecting their underdeveloped infants’ safety. Results suggest that at an age when an immature infant’s needs warmth, security, and protection, mothers with the s-allele exhibit insensitive behaviors that typically characterize future infant weaning. Other studies show that failure to restrain and protect an infant, and high rates of infant rejections are inappropriate when parenting a 3-month-old infant, and consequently may lead to aberrant infant development. These findings shed light on the etiology of variability in maternal warmth and care, and suggest that maternal 5-HTT genotype should be considered when assessing variation in mother-infant attachment behaviors and quality.